This invention relates to an X-ray mask for use in the X-ray lithography in semiconductor device process, and further it pertains to a process of producing the X-ray mask.
High-density, high-speed semiconductor devices, specially large scale integrated (LSI) circuits, which have been developed in recent years, inevitably require the microstructure of the components. In semiconductor device process, the wavelength of light, used in a photolithographic process, plays an important role, since the shorter the wavelength is, the tinier the component becomes. Soft X-rays (hereinafter referred to just as "X-rays") have a very short wavelength, in the vicinity of 1 nm. X-ray lithography, using such an X-ray source, is most attractive as a next generation lithographic system, accordingly.
Generally, an X-ray mask for use in the X-ray lithography employs a thin X-ray transmission film composed of a light element material for minimizing damping that occurs as an X-ray passes through the X-ray mask, and an X-ray absorption film deposited on the X-ray transmission film composed of a heavy metal with an X-ray adsorbability for transferring patterns (i.e., the absorber patterns) onto a semiconductor substrate.
With reference to the accompanying drawings, the aforesaid conventional X-ray mask is now described below. FIG. 33 shows, in cross section, a conventional X-ray mask G. In the figure, the reference numeral 1 denotes a frame-like mask base of a Si substrate. Formed on the surface of the base 1 is an X-ray transmission film 2 (a membrane) of a SiN film. On the surface of the X-ray transmission film 2 are LSI patterns 3. Also, alignment marks 4 are formed on the X-ray transmission film 2. Both the LSI pattern 3 and the alignment mark 4 are X-ray absorbers of tungsten films.
The X-ray mask G is now detailed. The X-ray transmission film 2 with a thickness of 2 .mu.m is formed on the surface of the base 1. On the surface of the X-ray transmission film 2 are the LSI pattern 3 and the alignment mark 4 with a thickness of 0.7 .mu.m. Further, an etching is applied to the back of the base 1 within an exposure region for X-ray transmission.
The X-ray transmission film 2 may be composed of a Si film or a thin diamond film in place of a SiN film. As for the LSI pattern 3 and the alignment mark 4, a thin heavy metal film of Au, Ta or the similar materials may be used instead of using a tungsten film.
The X-ray lithography based on X-ray sources, however, suffers a drawback in that converging lenses for X-rays are nonexistent, because of which the step-and-repeat projection with demagnification is difficult to perform. As a result of this reason, a full-scale, vary tiny transfer pattern with the same size as an image pattern on a semiconductor substrate must be formed on the X-ray mask G.
For the foregoing X-ray lithography, the X-ray mask G is placed proximal to a semiconductor substrate 30 having thereon alignment marks 31 and a resist 32 on its surface, with a gap of, for example, about 20 .mu.m between them (see FIG. 34). Then the X-ray mask G is aligned with respect to the semiconductor substrate 30. X-rays (synchrotron orbital radiation (SOR)) given off from an accumulation ring (not shown) and directed by beam lines are irradiated.
The X-ray lithographic system described above employs a detecting method for detecting possible misregistration between the X-ray mask G and the semiconductor substrate 30. In such a misregistration detecting method, the alignment mark 4, of a diffraction grating, formed on the X-ray mask G and the alignment mark 31, of a diffraction grating, formed on the semiconductor substrate 30 are at the same time irradiated with laser light beams (i.e., alignment light beams). Then the reflection diffracted light beam from the alignment mark 4 is compared with another from the alignment mark 31, which has been known as one of the most accurate alignment methods. This method will be detailed below.
As shown in FIG. 34, a laser light beam 13A incident upon the back of the X-ray mask G passes through the X-ray transmission film 2 so that the alignment mark 4 is irradiated. This results in the generation of reflection diffracted light beams. A first-order reflection diffracted light beam 14 of the reflection diffracted light beams is detected with a first photodetector (not shown). A laser light beam 13B, on the other hand, passes through a region of the X-ray transmission film 2 without the alignment mark 4 and irradiates the alignment mark 31 on the semiconductor substrate 30. This results in the generation of another first-order reflection diffracted light beam 15. The first-order reflection diffracted light beam 15 from the alignment mark 31 on the semiconductor substrate 30 is detected by a second photodetector (not shown). The first-order reflection diffracted light beam 14 is compared with the first-order reflection diffracted light beam 15 for phase difference, in order to detect a misregistration between the X-ray mask G and the semiconductor substrate 30.
However, in accordance with the above-described misregistration detecting method which uses the conventional X-ray mask G, a zero-order transmission diffracted light beam 16 is reflected by the surface of the semiconductor substrate 30 so that it again irradiates the alignment mark 4 to form an unwanted diffracted light beam 17 composed of a first-order transmission diffracted light beam. The trouble is that the unwanted diffracted light beam 17 diffracts with the first-order reflection diffracted light beam 14 necessary for detecting a misregistration. This results in the superposition of the first-order reflection diffracted light beam 14 and the unwanted diffracted light beam 17. The superimposed light beams are detected by the first photodetector. Considerable detection errors are likely to occur in detecting the first-order reflection diffracted light beam 14.
Additionally, the conventional X-ray mask G presents a further problem that detection errors may be caused not only by the aforesaid zero-order transmission diffracted light beam 16 but also by another first-order transmission diffracted light beam 18 or a high-order transmission diffracted light beam (not shown).
Further, the generation of transmission light beams that penetrate the X-ray transmission film 2 means that the first reflection diffracted light beam 14 used for the detection of misregistration has lower light strength compared to the laser light beam 13A irradiated upon the alignment mark 4. This results in a poor signal-to-noise (S/N) ratio.
Japanese published Patent Application 2-293748 discloses a technique to prevent an alignment light beam irradiated on the alignment mark 4 from passing through it (the alignment mark 4) as well as from finally reaching the semiconductor substrate 30. This prior art technique shows an X-ray mask with an alignment mark having on its surface an alignment light absorption film of colored gelatin. Such a proposed X-ray mask, however, has a disadvantage in that the light strength of a first-order reflection diffracted light beam used for detecting a misregistration cannot be intensified because the alignment light absorption film absorbs the alignment light beams. The prior art technique does not contribute to the improvement of S/N ratio.